Polycarbonates are a transparent plastic with excellent heat resistance, and are used in many applications as automotive parts including headlamp lenses, electrical and electronic parts, medical parts, housing materials, etc. In particular, use of polycarbonates as optical materials, e.g., compact-disk substrates and optical lenses, or automotive windowpane materials for an attractive appearance or weight reduction has come to be investigated recently. However, since polycarbonates have a large wavelength dependence of refractive index and are highly birefringent, uses thereof are often limited especially in optical applications. Automotive windowpanes made of polycarbonates have a severe problem that distortion of vision tends to occur due to their high birefringence. In addition, polycarbonates have many drawbacks including low rigidity, low surface hardness, and hence low marring resistance.
Acrylonitrile-styrene copolymers, which are an inexpensive transparent resin, are used in many fields including electrical appliances, vehicle parts, stationary, and miscellaneous goods. However, since acrylonitrile-styrene copolymers also are highly birefringent and have drawbacks similar to those of polycarbonates, uses thereof as optical parts are limited. In addition, acrylonitrile-styrene copolymers have low heat resistance and are hence unsuitable for use as an automotive windowpane. To improve the heat resistance of acrylonitrile-styrene copolymers, copolymerization with a maleimide compound was investigated. However, the copolymers with a maleimide compound not only are brittle and unsatisfactory in mechanical properties, but have a drawback of having a color. In U.S. Pat. No. 4,374,951 is disclosed a compatible resin composition comprising an acrylonitrile-styrene copolymer and a styrene-phenylmaleimide copolymer. However, this composition also has drawbacks of having a color and being brittle, though the heat resistance of the acrylonitrile-styrene copolymer has been improved. On the other hand, maleimide-olefin copolymers have satisfactory transparency, high heat resistance, and excellent mechanical strength, but a further diminution in birefringence is desired.